DLR1950 -
Congrats!!! You are gonna LOVE it! I agree, the 4369 layout is fantastic. We have had our '15 DS 4369 since mid-June and we could not be happier. Great choice, smart investment. We also have the two recliners -- a very comfortable and efficient plan.
The DEF is really nothing to worry about. You get a lot of mileage with a tank -- so much so that you really almost forget it's there. The only way we pay attention to it is to see the green LED lights begin to go down on the dash. You should always keep it above 1/2 so as not to risk the designed slowdown of the coach if the level drops too much but you will go a long time to get to 1/2 tank. Best estimate it is at least 1,500-2,000 miles before you need to add.
Can't answer the mileage question right now as we have not had enough experience or miles on the new coach to know accurately. We routinely got 8.1 to 8.3 on our '13 DS 4018 but I think we were pretty lucky as by most reports this was fantastic mpg. I do not expect that with the 4369 even though it has the same 450HP motor. Certainly, speed influences the mileage. We normally set the cruise at 71-72 mph. Many do no more than 65 mph and that seems to be a "sweet spot" with this coach.
Happy Birthday!! What an outstanding birthday present!
May you have several years of great motoring with your new coach.
Deek